The present invention refers to gyros and particularly to ring laser gyros. More particularly, the invention pertains to multiple gyro inertial measurement units which typically have three gyros, one in each coordinate direction. Such units also typically contain an accelerometer in each coordinate direction. Further, the invention pertains to gyro sensor orientation in an elongated, tubular or cylindrical inertial measurement unit.
Ring laser gyros function with counter propagating light waves in a ring cavity. The cavity has mirrors with quasi-total reflectivity and one output mirror with a small transmissivity. The two counter propagating beams are admitted through the output mirror. At rest, the emitted frequencies (or wavelengths) are equal, since the cavity length is the same in both directions. When it is rotated there is a small difference of light path lengths in the cavity because of the Sagnac effect, which yields a frequency difference between both counter propagating beams, as shown by the following formula: EQU .DELTA..function..sub.R =4.multidot..sup..alpha. /.sub..lambda.B .multidot..OMEGA.;
where A is the area enclosed by the ring cavity, B is the perimeter, and .lambda. is the wavelength of the light when the gyro is at rest. The frequency difference is measured by combining the two output beams to get an interference. Since the beams have different frequencies, their phase difference varies as EQU .DELTA..PHI.=2.tau..DELTA.F.sub.R .multidot.T.
The interference and intensity is modulated at the beat frequency .DELTA.F.sub.R which is EQU I=I.sub.1 1[1+cosine(2.tau.).DELTA.F.sub.R T].
The counting of the beats gives the rate of rotation, since a .DELTA. F.sub.R is proportional to the rotation rate .OMEGA.. The angle value corresponding to one modulation period is called the angular increment .THETA..sub.INC, with .THETA..sub.INC .multidot..DELTA.F.sub.R =.OMEGA.or .THETA..sub.INC =.lambda.B/4A.
Many high performance ring laser gyros have a triangular cavity with a perimeter of 20-30 centimeters. They typically function at a wavelength of 633 nanometers with an He-Ne amplifying media. .THETA..sub.INC is approximately equal 10.sup.-5 radian which is approximately equal to 2 arc seconds. A rotation of one degree per hour (i.e., 1 arc second/second) gives a beat frequency of 0.5 hertz.
The effect can be understood by considering an ideal circular cavity. Both counter propagating beams create a standing wave with a space of .lambda./2 between nodes. When the gyro is rotating, the standing wave remains at rest in inertial space, but the detector rotates and gives one count each time it is passing at a length of .lambda./2. Thus, the angular increment, .THETA..sub.INC, is simply .THETA..sub.INC =.lambda./2R, where R is the radius of the cavity (which is consistent with the general formula .THETA..sub.INC =.lambda.B/4A, since in this case B=2.tau.R and A=.tau.R.sup.2).
The main problem of the ring laser gyro is the phenomenon of mode locking between the counter propagating beams. As a matter of fact, these are oscillators with a very high resonance frequency (in the range of 5.times.10.sup.14 hertz) and a very small frequency-difference. There is some weak coupling between both oscillators. They get locked together and oscillate at the same frequency, creating a dead zone at low rotation rate. The main source of coupling is the back scattering of the mirrors. To solve this problem, the quality of the reflective coating on mirrors was drastically improved. However, even with very low scattering mirrors, there is still a dead zone (typically several degrees per hour) which is much wider than the potential sensitivity of the device. This lock-in is solved with a mechanical dither to vibrate the gyro in an oscillatory rotation about the rotation sensing axis at a rate outside of the dead zone. Present dithered laser gyros have excellent performances (i.e., bias stability better than 10.sup.-2 degree/hour and a scale factor accuracy better than one part per million over a dynamic range of +/-400 degrees/second).
Thus, one has ring laser gyros with the dithering mechanism which must be secured to a mount so there is significant inertial resistance to shift the gyro back and forth effectively. Many stand-alone packages, especially elongated housings, for gyros do not provide a sufficient mounting to effectively dither the ring laser gyro to attain the needed accuracy of the gyro.
The ring laser gyroscope and its aspects of dithering are elaborated in U.S. Pat. No. 3,373,650, entitled "Laser Angular Rate Sensor," by J. Killpatrick and issued Apr. 2, 1965, which is incorporated herein by reference; and U.S. Pat. No. 5,329,355, entitled "Dither Stripper to Leave Base Motion," by J. Killpatrick and issued Jul. 12, 1994, which is incorporated herein by reference.